Synthesis and properties of sulfonated poly(N-methylisatin-biphenylene) proton exchange membrane by superacid-catalyzed polymerization

A series of poly(N-methylisatin-biphenylene) polyelectrolytes were synthesized from trifluoroacetophenone and N-methylisatin by superacid catalyzed polyhydroxyalkylation reaction. Postsulfonation of this high molecular weight and thermochemically stable poly(N-methylisatin-biphenylene) with concentrated sulfuric acid resulted in homogeneous polyelectrolytes with controllable ion content (IEC: 1.43-2.39 meq./g). All these membranes were casted from dimethylsulfoxide (DMSO). The structural properties of the synthesized polymers were investigated by 1H NMR spectroscopy. The membranes were studied by ion exchange capacity (IEC), water uptake and dimensional stability comparing with Nafion. The proton conductivities of the membranes were in the range of 49.4-67.3 mS/cm at 80 °C with 80% relative humidity. The polymers containing fluoro group without ether linkage on polymer backbone were chemically stable towards nucleophiles (H2O, hydrogen peroxide, hydroxide anion and radical).